Yo. Wilts - there's plenty of new stuff to help with - no point wasting your time replying to a passing visitor who hasn't been back since mid-March...securespark said:
Upgrading Cooker MCB
- Thread starter Phantasm
- Start date
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Yup - but you could turn them all on at once - your description applies to how they behave once they are at the working temperature you want, but during warm-up, which could be quite long if there are pans etc to heat up, not just rings etc, you will see the full load current.Damocles said:
Which will overload the cable, and you are not supposed to rely on the protective device to prevent overloads.
Fine to use that diversity calculation when assessing the total demand, but risky down at the individual circuit level...
Well, there are two schools of thought on the applicability of the rule. I would not be surprised to find that most cooker supplies rely upon diversity rather than maximum load.
As to whether you might turn on everything at once, that would mean putting on four saucepans of cold water and both ovens at exactly the same time. How often do you do that? I never have. Maybe two pans at once while I go off and chop up some vegetables (ahem). One oven getting hot in advance? Most likely the oven will be turned on well ahead of the rings, because it will need a different cooking time. This is a domestic situation we are talking about here. If you have meals for 20 every day, then maybe I would think again.
Well, how on earth do socket rings work then? What happens if you plug in ten electric fires at once? It is assumed that you will not do this, but if you do the MCB/fuse will disconnect before any harm is done.
Cabling IS designed to work with protective devices. It is expected to get warm in use, and to to work satisfactorily and safely in the event of small overloads, up to the point at which the mcb trips. A 40A mcb may work happily at 50A for 10 minutes.
As to whether you might turn on everything at once, that would mean putting on four saucepans of cold water and both ovens at exactly the same time. How often do you do that? I never have. Maybe two pans at once while I go off and chop up some vegetables (ahem). One oven getting hot in advance? Most likely the oven will be turned on well ahead of the rings, because it will need a different cooking time. This is a domestic situation we are talking about here. If you have meals for 20 every day, then maybe I would think again.
Well, how on earth do socket rings work then? What happens if you plug in ten electric fires at once? It is assumed that you will not do this, but if you do the MCB/fuse will disconnect before any harm is done.
Cabling IS designed to work with protective devices. It is expected to get warm in use, and to to work satisfactorily and safely in the event of small overloads, up to the point at which the mcb trips. A 40A mcb may work happily at 50A for 10 minutes.
Me too.Damocles said:
Me neither. Got gas anyway, and when replaced the plan is for a gas hob & electric oven, but whatever, my habits would remain the same. And I always use the kettle if I want a pan of boiling water - more efficient. But it's not mega-rare for me to have 3 or 4 rings and the oven on the go - OK, not flat out, but who's to say for how long they would be if, unlike a gas hob, you need to wait for them to get hot? Finally, let's not forget self-cleaning ovens, which run very hot (I suspect flat out) for a substantial time.
Socket circuits are different from ones designed to supply 1 or 2 fixed items.
Yup - and if that 10 minutes happens 3 times a day for 25 years, what does it do to the cable? What I meant by the above is that you are not supposed to rely on the protective device to limit the load. It, and the cable, should be sized to allow the design current, and it should be rated so that it will protect the cable if something goes wrong, but that's it. Ib <= In <= Iz. If ever you find yourself saying (at the circuit level) "it doesn't matter if everything is turned on at once because the MCB will trip" then you've got a bad design.
Cookers are a very vexed topic, because they are naturally diverse items, and maybe I take an unnecessarily purist view, but if it were down to me I would not permit the use of diversity calculations a the circuit level.
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Then I take it BAS, you will not be fitting any socket rings in your house. Everything will be 16 or 20A radials, with just one double socket on the end. Just in case someone comes along and uses each one to it's maximum capacity. But if you are not going to permit diversity calculations on the circuit level, then I guess traditional multiple socket circuits are out.
OK, this is slightly different. Because in a ring, the more sockets there are, on the whole the lower the load is likely to be on any one socket. Since if there are not enough sockets, people use adapters and plug boards which multiply the load on each one. Mind you, the more cooker rings you have, the less likely it must be that you will be using all of them at once.
Now as to the cabling. I take the view that circuits are designed to work, and to work safely in all circumstances. I assume that if a certain cable is specified to be safe to use with a certain breaker, then it really is safe. Further more, I think there is good evidence of this, because houses do not regularly burn down due to normally operating circuits, even when they are loaded to their limit. The reason most quoted for getting rid of rewirable fuses is to prevent people fitting extra big fuse wire, which really would allow the cables to be overloaded and dangerous.
As to overloading them 10 minutes, three times a day, I think they should do fine. Breakers and fuses are all thermal devices, They rely on getting hot to disconnect the circuit. Circuit designers know the breaker will allow a nominal overload for a certain time, and I do not think you should really consider this as an overload. Rather, operating at the design limit.
I am assuming here that if someone finds themselves in the position where their breaker is tripping regularly, or even occasionally, they will find out why. This is one reason I hate breakers which trip when bulbs blow. It gives people completely the wrong idea about it being safe just to reset a breaker. It is not.
A cooker is not a fixed item in the same way as an immersion heater, shower or 100 ton winch is. Such things are very likely to use their full power continuously. It is quite unusual to run a cooker flat out. The diversity rule in fact assumes that part of the cooker will be operating full power. Take the first 10A, plus 1/3 the remainder. That might be a couple of rings full power and the rest running on simmer. Very realistic to real life use of a cooker.
Someone suggested that the actual life expectancy of PVC cabling is unknown. It has not been used long enough to tell. But 100 years was suggested as quite possible. So maybe this will be degraded by running it at maximum. Still pretty good and with a decent safety marging for someone coming round every few decades to check it.
There is a sensible limit on how far down the chain you should apply diversity. But you can not ignore it when considering devices drawing large current compared to the available supply. Otherwise installing one shower and one cooker would mean you had to install three phase in every house.
And finally, Several people have suggested using a larger size cable on cooker or showers than strict limits might suggest. Thats fine with me. Arguably even cost effective. I'm not saying you should do everything to the minimum standard. Just that you do not need to rip out a circuit which actually conforms to power requirements and has been demonstrated to be safe.
OK, this is slightly different. Because in a ring, the more sockets there are, on the whole the lower the load is likely to be on any one socket. Since if there are not enough sockets, people use adapters and plug boards which multiply the load on each one. Mind you, the more cooker rings you have, the less likely it must be that you will be using all of them at once.
Now as to the cabling. I take the view that circuits are designed to work, and to work safely in all circumstances. I assume that if a certain cable is specified to be safe to use with a certain breaker, then it really is safe. Further more, I think there is good evidence of this, because houses do not regularly burn down due to normally operating circuits, even when they are loaded to their limit. The reason most quoted for getting rid of rewirable fuses is to prevent people fitting extra big fuse wire, which really would allow the cables to be overloaded and dangerous.
As to overloading them 10 minutes, three times a day, I think they should do fine. Breakers and fuses are all thermal devices, They rely on getting hot to disconnect the circuit. Circuit designers know the breaker will allow a nominal overload for a certain time, and I do not think you should really consider this as an overload. Rather, operating at the design limit.
I am assuming here that if someone finds themselves in the position where their breaker is tripping regularly, or even occasionally, they will find out why. This is one reason I hate breakers which trip when bulbs blow. It gives people completely the wrong idea about it being safe just to reset a breaker. It is not.
A cooker is not a fixed item in the same way as an immersion heater, shower or 100 ton winch is. Such things are very likely to use their full power continuously. It is quite unusual to run a cooker flat out. The diversity rule in fact assumes that part of the cooker will be operating full power. Take the first 10A, plus 1/3 the remainder. That might be a couple of rings full power and the rest running on simmer. Very realistic to real life use of a cooker.
Someone suggested that the actual life expectancy of PVC cabling is unknown. It has not been used long enough to tell. But 100 years was suggested as quite possible. So maybe this will be degraded by running it at maximum. Still pretty good and with a decent safety marging for someone coming round every few decades to check it.
There is a sensible limit on how far down the chain you should apply diversity. But you can not ignore it when considering devices drawing large current compared to the available supply. Otherwise installing one shower and one cooker would mean you had to install three phase in every house.
And finally, Several people have suggested using a larger size cable on cooker or showers than strict limits might suggest. Thats fine with me. Arguably even cost effective. I'm not saying you should do everything to the minimum standard. Just that you do not need to rip out a circuit which actually conforms to power requirements and has been demonstrated to be safe.
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